This is the first "chapter" in a not-quite-book I've started working on - I have an introductory post about that here: https://simonwillison.net/2026/Feb/23/agentic-engineering-pa... The second chapter is more of a classic pattern, it describes how saying "Use red/green TDD" is a shortcut for kicking the coding agent into test-first development mode which tends to get really good results: https://simonwillison.net/gu…
I also see that the tests generated by ChatGPT are far too few for the code features implemented. The cannot be the result of actual red/green TDD where the test comes before the feature is added.
For examples, 1) the code allows "~~~" but only tests for "```", 2) there are no tests when len(fence) fence_len, and 3) there are no tests for leading spaces.
There's also duplicate code. The function _strip_closing_hashes is used once, in the line:
text = _strip_closing_hashes(m.group("text")).strip()
def _strip_closing_hashes(s: str) -> str:
s = s.rstrip()
# remove trailing " ###" style closers
s = re.sub(r"[ \t]+#+\s*$", "", s).rstrip()
return s
I think that rstrip() should be replaced with a strip(), the function renamed to "_get_inline_content", and used as "text = _get_inline_content(m.group("text")).
Also, the Google spec also says "A sequence of # characters with anything but spaces following it is not a closing sequence, but counts as part of the contents of the heading:" so is it really correct to use "\s*" in that regex, instead of "[ ]*"? And does it matter, since the input was rstrip'ped already?
So perhaps:
def _get_inline_content(s: str) -> str:
s = s.rstrip(" ") # remove trailing spaces
s = s.rstrip("#") # removing "#" style closers
return s.strip() # remove leading and trailing whitespace